1. Field of Invention
This invention relates to surgical procedures and instruments and, more particularly, to method and apparatus for measuring the flow of blood through a vessel by observing the change in blood pressure as it flows past a balloon catheter, and the like.
2. Description of the Prior Art
To measure blood flow in veins and arteries, a bolus of cool, normal saline solution is injected into the vessel under observation. The bolus, injected at a temperature that is lower than the blood temperature, mixes with the blood, and the temperature of the mixture is sensed through multiple thermistor catheters. The observed temperature increase in the bolus is related, in general, to the blood flow in terms of, for example, milliliters per minute. The temperature change, however, is also relates to a number of other factors that are entirely independent of the blood flow. For example, the temperature of the bolus will change between the injection site and the thermistors; the temperature difference between the blood and the injectate can vary; and the volume of the injected bolus also can vary. These, and possibly other effects combine to produce errors in the accuracy of the flow measurement that can be as great as .+-.10%.
Not only is this prior art technique inaccurate, but it also requires a minimum of three injections to produce an average value of blood flow that will provide this .+-.10% accuracy. Inaccuracy aside, perhaps the most undesirable feature of this prior art technique is the risk to the patient inherent in injecting foreign material into the circulatory system.
Unquestionably, there is a need for a more accurate blood flow measuring technique that does not required extensive surgery or injecting foreign matter into the pateint.